Process for producing middle distillates by hydroisomerizing and hydrocracking feeds from the fischer-tropsch process

A process for producing middle distillates from a paraffinic feed produced by Fischer-Tropsch synthesis, said process comprising subjecting said paraffinic feed to a catalytic hydrocracking/hydroisomerization with a hydrocracking/hydroisomerization catalyst which comprises at least one group VIB or group VIII hydrodehydrogenating element, 0.2% to 2.5% by weight of an oxide of a phosphorus doping element, and a non-zeolitic support based on silica-alumina containing a quantity of more than 5% by weight and 95% by weight or less of silica (SiO2), said catalyst having the following characteristics:

• a mean pore diameter, measured by mercury porosimetry, in the range 20 to 140 Å;
• a total pore volume, measured by mercury porosimetry, in the range 0.1 ml/g to 0.5 ml/g;
• a total pore volume, measured by nitrogen porosimetry, in the range 0.1 ml/g to 0.5 ml/g;
• a BET specific surface area in the range 100 to 550 m2/g;
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 140 Å, of less than 0.1 ml/g;
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 160 Å, of less than 0.1 ml/g;
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 200 Å, of less than 0.1 ml/g;
• and
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 500 Åof 0.05 to less than 0.1 ml/g
• an X ray diffraction diagram which contains at least the characteristic principal peaks of at least one transition alumina included in the group composed of alpha, rho, khi, eta, gamma, kappa, theta and delta aluminas;
• and
• a catalyst settled packing density of more than 0.75 g/cm3.